A withered biwa player unfolds a captivating tale from 900 years ago, detailing the epic quest of the Genji clan as they sought the Imperial Regalia to unite the emperor's throne. Following their triumph over the rival Heike clan at the Battle of Dan-no-ura, a tragic fate befalls the child emperor, who drowns while clutching the Grasscutter Sword. Fast forward 300 years, agents of Ashikaga Yoshimitsu (Tasuku Emoto) recruit a young boy named Tomona (Mirai Moriyama) and his father for a perilous task: retrieving a box from a Heike shipwreck. Inside, they discover a sword that holds dire consequences. Unaware of its true identity as the Grasscutter Sword, Tomona's father unsheathes it, unleashing a devastating energy that claims his life and blinds Tomona in the process.
What follows is an arduous journey, as Tomona seeks the truth behind that fateful day, accompanied by the ghost of his father. During his travels, he encounters a troupe of blind biwa players, inspiring him to learn the instrument and become part of their ensemble. Embracing his new identity, he adopts the name "Tomoichi," although this transformation complicates his father’s spirit's search for him.
Parallel to Tomona's journey is that of a Noh dance troupe leader, who wears a demonic mask. The unfolding narrative sees a horrifying string of murders of biwa players, alongside the birth of the leader’s third son, a deformed child with three stunted limbs, an elongated arm, and scales covering his skin. Harboring a deep loathing for his son, the leader treats him cruelly—forcing him to wear a mask and behave like a pet. Yet, one day, as the deformed son observes his father teaching Noh dance, he secretly practices alongside his siblings, leading to a miraculous restoration of his legs.
Wandering through the village, the deformed boy crosses paths with Tomoichi. Their bond flourishes, untainted by appearances, as the boy reveals his chosen name "Inu-Oh," meaning "Dog King." Tomoichi shares with Inu-Oh the vision of his father's spirit, which leads him to recognize the multitude of Heike warriors' spirits surrounding Inu-Oh, each eager to tell their own stories. This revelation ignites their desire to create a new performing troupe, with Tomoichi rebranding himself as "Tomoari."
The duo’s significant debut on a bridge captures the attention of onlookers, as Tomoari showcases his hair metal-inspired performance while Inu-Oh dances beneath, dramatically narrating the plight of the Heike soldiers. Their act becomes an overnight sensation, propelling both Tomoari and Inu-Oh to fame.
However, their rising popularity does not go unnoticed by Ashikaga, who craves only his clan's rendition of the Heike tales. In a twisted plot to ignite jealousy, he manipulates Inu-Oh's father with promises of wealth, inciting conflict that culminates in the troupe’s performance unveiling Inu-Oh's tragic origins—the deal his father made with the demon mask for renown, which required the lives of biwa players and the innocence of his unborn child. The tragic consequence of this pact spurs the spirits of the Heike to flock to Inu-Oh, empowering his ascent while ultimately leading to his father’s demise when he attempts to betray him.
As Inu-Oh’s face is finally restored, tragedy looms again when Ashikaga’s forces quell the troupe. Despite Inu-Oh's concessions to perform a censored version of Heike history, Tomoari defiantly continues to share the truth, resulting in his execution. In the wake of despair, Inu-Oh experiences fleeting fame as a Noh dancer, only to be forgotten posthumously.
In a poignant twist, the narrator reveals himself as Tomoari's spirit, tethered to the physical world until modern times, now reclaiming the name “Tomona.” Inu-Oh’s spirit appears, explaining his lengthy search for Tomona due to his name change, and together they embrace their shared past through performance once more, sealing their legendary bond.
